An analysis of the passwords leaked from LinkedIn shows that users need both complex and unique passwords. It's especially important in an era of cloud-based applications.

The leak of LinkedIn passwords has underscored that users of online services should choose unique and complex passwords for access to cloud services, according to an ongoing analysis.

Ever since the June 6 discovery of the theft, security consulting firm KoreLogic has worked on analyzing the list of passwords taken from the business-oriented social network and posted to an online forum. Like preceding analyses of leaked passwords, the company found that many people do not choose good passwords, a major flaw in a leak like LinkedIn's where the passwords can be easily decrypted by brute-force methods.

The list of password hashes leaked from social networks included some 6.5 million unique cryptographic hashes. Each hash represents a unique password and protects the secret by scrambling the strings of characters, numbers and symbols using an algorithm that is hard to reverse. In the case of LinkedIn, however, the company did not use an additional random input, or salt, that would have made the hashes much stronger. The result is that common patterns of passwords can be found quickly.

"When you log into a Website, you don't know if they are using MD5, you don't know if they are using SHA-1," said Rick Redman, senior security consultant with KoreLogic. MD5 is no longer considered a strong hashing algorithm, as ways of breaking the cryptographic hash have already been found. And SHA-1, which LinkedIn used, needs to be salted to maximize its security.

In total, nearly 80 percent of the passwords have been decrypted, said Redman. Users need to expect that their password will be leaked and take steps to protect all their accounts, he said. They should choose complex passwords to hinder brute-force attacks, and unique passwords so that one compromised account does not lead to another.

Data from users who change their passwords occasionally suggest that the leak of the list happened some six to seven months ago, he says. Attackers have had all that time to attack the list and find weak passwords that, if reused, could be exploited on other accounts.

The password list had some additional interesting characteristics, said Redman. While there were some 6.5 million hashes, because many people use a number of common, weak passwords, some of the hashes may represent multiple users.